The present invention relates to a synthetic aperture radar (SAR) and, more particularly, to an along-track interferometric SAR having a single SAR channel and performing observation only once.
Mounted on a satellite, aircraft or similar mobile platform, an SAR transmits a radio wave toward the ground on the side with respect to the direction of movement in order to acquire bidimensional data for reconstructing the image of the ground. The SAR is taught in Japanese Patent Laid-Open Publication Nos. 62-3676 and 62-8081 by way of example.
An SAR is applicable also to the remote detection of the velocity of an object moving on the ground. This kind of application is known as an along-track interferometric SAR and discussed in, e.g., Richard E. Carande "Dual Baseline and Frequency Along-Track Interferometry", '92, IEEE, pp. 1585-1588. The along-track interferometric SAR has two SAR channels adjoining each other in the direction of movement and observes a single target two times at slightly different times, thereby producing two SAR images. The phase difference of the target point is measured on the basis of the two SAR images (interferometric processing) so as to determine the velocity of the target in the direction parallel to the SAR eye direction, i.e., the component of the velocity perpendicular to the azimuth direction. While an SAR is usually operated as a single-channel SAR, its antenna may be divided in the direction of movement and operated as two-channel SAR, i.e., an along-track interferometric SAR (needing two SAR channels). Further, a single-channel SAR may be used to perform observation two times. Specifically, an along-track interferometric SAR can be implemented if a single-channel SAR observes a target once, then returns, and then observes the target on the same course again.
The implementation of the interferometric SAR is disclosed in, e.g., Japanese Patent Laid-Open Publication No. 7-110377. However, the interferometric SAR taught in this document is a cross-track interferometric SAR for acquiring altitude data. Although the processing scheme of a cross-track interferometric SAR resembles the processing scheme of an along-track interferometric SAR, the former is different from the latter as to the kind of data to deal with and the purpose. A cross-track interferometric SAR reconstructs two SAR images based on SAR data observed on different flight courses and thereby acquire the altitude data of a target; the two SAR images are different from each other as to the positional relation to the ground surface. On the other hand, an along-track interferometric SAR reconstructs two SAR images based on SAR data observed on the same flight course at different times and thereby acquire the velocity data of a target; the two SAR images are different from each other as to the time of observation. In a broad sense, an interferometric SAR includes both of a cross-track interferometric SAR and an along-track interferometric SAR, but in a narrow sense it refers to a cross-track interferometric SAR.
The conventional technologies discussed above have some problems left unsolved, as follows.
(1) The along-track interferometric SAR using two SAR channels produces two SAR images deviated in time from each other. Such two-channel SARs cannot implement an along-track interferometric SAR without increasing weight and power consumption, and without needing a high data rate. PA0 (2) The along-track interferometric SAR implemented by a single antenna divided in the direction of movement needs only a single antenna, but two independent channels must follow the antenna. This also increases weight, power consumption and required data rate, and in addition complicates the SAR system. PA0 (3) The method observing a target two times by use of a single-channels SAR is not practicable unless the flight course of an aircraft or a satellite loaded with the SAR is controlled with utmost accuracy in order to bring two times of flight into coincidence as to the course. Moreover, the considerable time lag between the two times of observation severely limits the upper limit of velocity of the target which can be observed.